1. Field of the Invention
The present invention relates to a cryogenic refrigerator of the refrigerant-filled or refrigerating force storage chamber type (regenerator) and, more particularly, it relates to refrigerants filled in a multi-stage cylinder and a seal assembly capable of sealing the space between the cylinder and a piston without using any lubricant.
2. Description of the Related Art
Cryogenic refrigerators of various types are now on the market. The Gifford-McMahon type cryogenic refrigerator is typical of these refrigerators of the refrigerant-filled chamber type. This cryogenic refrigerator of the refrigerant filled chamber type has a cycle. According to the cycle, compressed helium gas is cooled while passing through the refrigerant-filled chamber, expanded to create cold atmosphere at low temperature area, then reversed through the refrigerant-filled chamber to cool the refrigerant therein and finally collected.
The cryogenic refrigerator often has the refrigerant-filled chamber at each of plural stages arranged from normal to low temperature area. Lead refrigerant having high specific heat at low temperatures is usually used in a refrigerant-filled chamber located on the lowest temperature side or at the final stage. Further, the other conventional cryogenic refrigerators of the refrigerant-filled chamber type also have the refrigerant-filled chamber at each of plural stages arranged from normal to low temperature area. Lead refrigerant having high specific heat at low temperatures is also used in the refrigerant-filled chamber located on the lowest temperature side or at the final stage.
However, the above-described cryogenic refrigerators of the refrigerant-filled chamber type in which the lead refrigerant is used in the final stage refrigerant-filled chamber have the following problem. Lead is a material having high specific heat capacity at low temperatures, but when temperature becomes lower than 15 K., its specific heat quickly decreases with the lowering of temperature. When lead is used as the refrigerant in the final stage chamber, therefore, the lowering of heat exchanging efficiency makes it difficult to create cold atmosphere lower than 10 K.
In order to solve this problem, Er.sub.3 Ni having a specific heat capacity larger than that of lead at a temperature range lower than 10 K. has attracted attention, these days, as as the refrigerant filled in the final stage chamber.
when Er.sub.3 Ni which is a magnetic material is filled in the final stage chamber, temperature can be lowered to 4 K. maximum and a refrigerating capacity of about 0.1 W can be attained at 4.2 K(or boiling point of liquid helium).
Even in the case of the cryogenic refrigerator in which Er.sub.3 Ni is filled in the final stage chamber, however, its refrigerating capacity attained at about 4.2 K. is still lower than that actually needed.
Further, it is not desired in some of those refrigerators which use the cylinder and the piston that lubricant (oil etc) is used in the seal assembly. As being typical of these refrigerators, the cryogenic refrigerator in which helium gas is used as coolant can be mentioned.
when lubricant is used at sealing points in this cryogenic refrigerator, helium gas is contaminated and the malfunction of the refrigerator is caused, as well, because the lubricant is frozen. Therefore, no lubricant can he used in the refrigerator.
As shown in FIG. 25, therefore, a clearance 103 between a cylinder 101 and a piston (or displacer) 102 is sealed by a seal assembly 104 of the non-lubricant type in the case of the conventional cryogenic refrigerator, thereby preventing helium gas from being leaked.
As shown in FIGS. 26 and 27, a seal ring 106 is fitted in a annular groove 105 on the outer circumference of the displacer 102. The seal ring 106 is made of resin, having both ends, and it is pressed against the inner circumference of the cylinder 101 by spring ring 107 located inside the seal ring 106.
As shown in FIG. 28, both ends of the seal ring 106 have thickness-reduced portions 108 which are overlap each other.
In the case of the conventional seal assembly 104, however, the thickness-reduced portions 108 of the seal ring 106 closely contacted each other is loosened particularly in the circumferential direction of the cylinder 101, the amount of helium gas leaked through the thickness-reduced portions 108 of the seal ring 106 is thus increased, and temperature is raised at the cooling stage, when the seal ring is thermally shrunk at very low temperatures.
In the case of the conventional seal assembly 104, therefore, the refrigerating efficiency of the refrigerator greatly changes as temperature changes. It is therefore difficult to guess from performance tests of the refrigerator conducted under normal temperature what refrigerating capacity the refrigerator can have at very low temperatures.
when the conventional seal assembly 104 of the non-lubricant type is used particularly at very low temperatures, no sealing capacity can be obtained as desired and the refrigerating capacity of the refrigerator achieved has a limit accordingly.
Further, even when the seal assembly 104 is actually incorporated into the cryogenic refrigerator, the seal ring 106 is thermally shrunk. This causes a clearance between the seal ring 106 and the annular groove 105, thereby making it impossible to reduce the amount of gas leaked to an extent greater than a certain value.